Thin film transistors formed over a flat plate such as a glass substrate have been manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Thin film transistors manufactured using amorphous silicon have low field effect mobility but can be formed over a large glass substrate. On the other hand, thin film transistors manufactured using crystalline silicon have high field effect mobility, but due to a crystallization step such as laser annealing, such a transistor is not necessarily suitable for being formed over a large glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 discloses a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film, and such a transistor is used as, for example, a switching element of a liquid crystal display device.